JPQL 在 Select 语句中创建新对象 - 避免还是拥抱？

发布于 2024-08-23 12:03:19 字数 256 浏览 4 评论 0原文

我最近了解到可以在 JPQL 语句中创建新对象，如下所示：

select new Family(mother, mate, offspr)
from DomesticCat as mother
    join mother.mate as mate
    left join mother.kittens as offspr

这是应该避免还是应该拥抱的事情？根据良好实践，什么时候使用此功能是合理的？

原文

I've learnt recently that it is possible to create new Objects in JPQL statements as follows:

select new Family(mother, mate, offspr)
from DomesticCat as mother
    join mother.mate as mate
    left join mother.kittens as offspr

Is this something to be avoided or rather to embrace? When is usage of this feature justified in the light of good practices?

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猫腻 2024-08-30 12:03:19

不要避免它，SELECT NEW 之所以存在，是因为它有完全有效的用例，正如§10.2.7.2 中所提醒的那样。的 SELECT 子句中的 JPQL 构造函数表达式 EJB 3.0 JPA 规范：

构造函数可以用在
SELECT 列表返回一个或多个 Java
实例。指定的类不是
必须是一个实体或
映射到数据库。的
构造函数名称必须完整
合格。
如果指定了实体类名
在 SELECT NEW 子句中，
结果实体实例位于
新状态。
选择新 com.acme.example.CustomerDetails(c.id, c.status, o.count)
来自客户 c 加入 c.订单 o
WHERE o.count > 100

简而言之，当您不想以类型安全的方式（而不是 Object[]）检索完整的实体或完整的对象图时，请使用 SELECT NEW。将查询结果映射到实体类还是非映射类将取决于您的选择。一个典型的例子是列表屏幕（您可能不需要所有详细信息）。

换句话说，不要到处使用它，但不要禁止它的使用（很少有东西只是黑或白的）。

Don't avoid it, the SELECT NEW is there because there are perfectly valid use cases for it as reminded in the §10.2.7.2. JPQL Constructor Expressions in the SELECT Clause of the EJB 3.0 JPA Specification:

A constructor may be used in the
SELECT list to return one or more Java
instances. The specified class is not
required to be an entity or to be
mapped to the database. The
constructor name must be fully
qualified.
If an entity class name is specified
in the SELECT NEW clause, the
resulting entity instances are in the
new state.
SELECT NEW com.acme.example.CustomerDetails(c.id, c.status, o.count)
FROM Customer c JOIN c.orders o
WHERE o.count > 100

In short, use the SELECT NEW when you don't want to retrieve a full entity or a full graph of objects in a type safe way (as opposed to an Object[]). Whether you map the result of a query in an entity class or a non mapped class will depend on your select. A typical example would be a list screen (where you might not want all the details).

In other words, don't use it everywhere but don't forbid its use (few things are only black or white).

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